A known power seat slide device disclosed in, for example JP2000-158983A, includes a lower rail fixed to a vehicle floor, an upper rail slidably supported to the lower rail in order to support a vehicle seat, a nut member fixed to the lower rail, a screw shaft screwed into a nut member, which is rotatably supported by the upper rail, and a gear box supported by the upper rail and connected to the screw shaft and the electric driving source.
When the known device rotates the screw shaft relative to the nut member through the gear box by the electric driving source, the screw shaft is screwed into the nut member, and then the upper rail is moved so as to slide relative to the lower rail. Thus a position of the vehicle seat relative to the vehicle floor can be adjusted in a vehicle front-rear direction.
Further, the nut member, into which the screw shaft is screwed, is fixed to the lower rail by use of a bracket screwed to the lower rail with bolts so as not to slide in a longitudinal direction (a front-rear direction) of the rail.
According to the known device, when a load is applied to the upper rail in a vehicle front-rear direction, the load transmits from the upper rail, which is attached to the vehicle seat, to the screw shaft, witch is engaged with the upper rail. At this point, because the upper rail is limited so as not to slide relative to the lower rail by means of the nut member into which the screw shaft is screwed, the load applied in a vehicle front-rear direction transmits to the nut member through the screw shaft. Then, the load transmitted to the nut member is absorbed by the bracket, which is fixed to the lower rail in order to maintain the nut member. However, if the nut member is not supported by the bracket appropriately, the excessive load at the time of, for example the vehicle collision, transmits from the screw shaft to the gear box, as a result a bending stress may occurs on the screw shaft.
In order to secure the nut member appropriately, the thickness of the bracket has been set to be larger, a reinforcement has been welded to the bracket, and the bracket has been formed in a chambered-shape. However, these measures result in increasing the costs of the seat slide device, in addition, the welding may result in degradation on a dimensional accuracy due to heat distortion. The costs is further increased when the bracket is processed in the chambered-shape, and if such chambered-shaped bracket is provided within the rail, more space needs to be provided, as a result, the costs is further increased, in addition, the total weight of the power seat slide device has been increased.
A need thus exists to provide a vehicle power seat slide device, which maintains a nut member appropriately in a simple configuration, at the same time, the device can be downsized and reduced in weight.